A UV254 probe/sensor for measuring BOD. TOC, COD and DOC

What is UVT?

Ultraviolet transmittance refers to the amount of light that passes through a sample at the wavelength of 254 nm.

UV254 is absorbed and scattered by the organics, colloids, and suspended solids.

UV Absorbance often expressed as A254 or ABS is the light absorbed by a sample.

Absorbance and Transmission are related terms and are related logarithmically

How is UVT Measured?

Ultraviolet (UV) light, at the wavelength of 254 nm, is passed through a sample. This is compared to the light intensity in deionized water.

The result is given as a percentage transmitted %T = I/Io

Alternatively, this can be expressed as absorbance ABS = log10 ( 1 / %T )

A UV254 sensor measuring UVA/UVT in a water works
UV254 Analyser to Measure BOD (Biological Oxygen Demand)

How is concentration related to ABS/UVT?

The absorbance is related to the concentration by Beer lamberts law

ABS = L ε C

Where:

ABS - Absorbance
L - The path length
ε - Molar absorptivity
c - Concentration

What is SUVA? (Specific UltraViolet Absorbance)

SUVA is calculated by dividing the UVA value by the DOC (Dissolved organic content)

SUVA = ABS / DOC

SUVA is a method of normalising UV254 with the total organic content (TOC). This is needed as UV254 only measures aromatic content ​

uv254 device measuring SUVA with DOC correction
Plot of UV254 growth with BOD (Biological oxygen demand) concentration

How does UV254 relate to BOD, COD, TOC and DOC?

All aromatic surrogates concentration grow linearly with UV254 absorption. Doubling the surrogate concentration doubles the UV254 absorption


What is Turbidity Compensation?

Turbidity is the merkyness of water and this can interfere with your UV254 measurement.

UV254 is blocked/absorbed by particles aswell as organic content.

To compensate for the turbidity we use an infra-red light source to measure the absorption of the particles and subtract it from the UV measurement

The mathematics and science of turbidity compensation in written form
Anoxic tank at waste waterworks with uv254 analyser on the gantry

What absorbs at 254nm?

TOC, BOD, COD, and DOC all absorb light at a wavelength of 254nm. The proporties that this group of parameters have is the aromatic bonds that they contain.

These aromatic hydrogen bonds will absorb light that means we can know they are present by a drop in light intensity through the sample. Twice the aromatic bonds will have twice the uv254 absorption. Twice absorption means twice the concentration of organic material.